CRM250AR Top end refresh

[gregorious77]

So here is the thread that all AR owners have been waiting for...................... maybe.
After a whole load of hassle finding a std bore piston (thought to be unavailable) I found one and have it in my possession.
Wossner part number 8071D040 Dia 66.33mm clearance 0.07mm


I purchased it from Off Road Only but I think Tom at Leisure Trail (brilliant help with tech knowledge while I was sourcing this piston)* will get a couple to stock now he knows they are available again so you could order all of the required gaskets and piston from them.
*As was Knackered Mk1

As I knew this was coming I decided to start the strip down.

My bore had brown streaks down it and the piston had some blowby and the whole lot was covered in a fair bit of oil including the bottom end.

Bore (oily)


Brown streaks


Piston


Exhaust side (evidence of 4 corner seize)


Inlet side (evidence of 4 corner seize)

[gregorious77]

Side of piston


The Wossner is the same in all directions except the ring positions and pin detail.

[gregorious77]

Took the cylinder to work and cleaned it up in our parts washer then used scotchbrite in a criss cross pattern followed by electrical contact cleaner and clean rags and finished with a blow out using compressed air and finally WD40 and clean rags. NOTE - never hone a plated bore.
https://www.youtube.com/watch?v=vaFp8aYkHNA

Resulting bore is shiny and streaks are barely visible.






PV got the treatment too.


Head cleaned up with a rag and some WD40.

[gregorious77]

Measured a high calorie performance piston to see if it would fit.


Alas it was too big so an overbore and greaseproof paper lining would be required.


Needed silicon so rebuild only got as far as piston on rod tonight.

[mekros]

I'm interested to see what the high calorie piston rings look like.

[jim157]

I'm interested to see what the high calorie piston rings look like.

[mekros]

Let me know how the run in goes with that lot. lol

[admin]

epic post in true PB style. nice one fella. watching with interest

[admin]

what's the difference in weight between the 2 items ?

[gregorious77]

sorry didn't weigh them.
Ran her up last night though. sounded a bit knocky maybe a bit of pinking due to higher compression.
Ill see what she is like once run in.

[admin]

how is she running pal ? all ok i hope

[gregorious77]

Strong and fast but rattles like a diesel. Guessing clearances are a bit on the loose side so getting piston slap. Gonna run it like this and look at bore and plate +2mm later on.

[admin]

lol brilliant
so you reckon the plating was worn a bit too much or the piston just isnt as snug a fit as the OE one was ?

[gregorious77]

Plating looked good. There is a b and c sizes oe though so I guess it was the biggest size bore to start with. I have a spare barrel but it has a few dinks in the top sealing surface.

[gregorious77]

Purchased a whole engine for spares. New bore and piston with 5 miles on it. Failed newly rebuilt crank though. I will strip the crank and do my damndest to discover the origin of the illusive rod through accurate measurement and hours and hours of research.